1. Field of the Invention
The invention on which this application for patent is based encompasses the use of erosion-resistant silicone coatings for the protection of fluid-handling parts from erosion by impacting particles, impingement, and cavitation. The invention further encompasses methods of application, maintenance, and repair of these coatings.
2. Description of Related Art
Substrates such as the surfaces and interiors of machine or structural parts, often require protection against wear. A material which is selected for, say its resistance to breakage by brittle fracture may not have adequate resistance to one or more kinds of wear. A coating can then be applied to the exterior of the part, in order to protect the material forming the bulk of the part from the effects of wear.
A machine or structural part may suffer wear when it is continuously rubbed against another surface at high speeds. For example, a machine tool bit may be worn down through prolonged use. To reduce such wear, the bit is often coated with a hard material.
The high-speed impact of particles can also induce wear; this process of wear is a form of erosion. The erosion of rock by blown sand is well known. In this application for patent, the term erosion is used to refer to the wearing of a part by particle impact, impingement, or cavitation. For certain fluid-handling parts, the primary function of solid material which makes up certain regions of the part can be to occupy volume in order to either efficiently move the fluid or to efficiently convert movement of the fluid to another form of energy. Modification of the dimensions of a part or loss of the material of which a part is formed through erosion caused by particle impact, impingement, or cavitation affects the efficiency of moving a fluid or converting fluid motion to another form of energy. It is desirable to prevent the loss of material from or alteration of dimensions of a fluid-handling part so that the original shape, volume, and contours of the part are most nearly preserved.
Sheathing a fluid-handling part with a hard surface may not provide adequate or appropriate protection against erosion by high-speed particle impact. For example, a common problem with helicopter operation is erosion of rotors by impacting particles such as dirt, sand grains, and water droplets. This erosion may require the frequent replacement of expensive rotors, compromise aerodynamic performance, and in some cases lead to catastrophic failure of the rotor during helicopter operation. The problem of rotor erosion is of special concern to the military: operation in arid or desert environments may result in erosion at a rapid rate and the exigencies and uncertainties associated with combat may preclude regular maintenance. Presently, several approaches, none of which are fully satisfactory, are taken to protect helicopter rotors. In one approach, metal strips are fastened to the leading edge of the rotors. Metal strips are rigid and therefore compromise the aerodynamic performance of composite rotors which are designed to flex in several modes; the metal strips may place extra mechanical stress on the rotors, for example, by constraining their flexing. The metal strips can initiate small cracks in the composite material of the rotor; these cracks can then grow, resulting in catastrophic failure. Because of the problem of crack initiation, frequent, expensive inspection is required. Furthermore, the metal strips are rapidly damaged by impacting particles. Hard, brittle metal strips tend to have material chipped off by the particles and softer metal strips tend to suffer deformation.
Attempts to protect helicopter rotors have also included the use of polyurethane tape applied to the leading edge of rotors. Because the tape is flexible, it has the advantage over the metallic strips of not impeding the flexing of a composite rotor. However, the tape can trap sand beneath it, which can compromise the mass balance of rotors on opposite sides of the drive shaft and affect performance. Furthermore, the tape is rapidly abraded by impacting sand and rain droplets and requires frequent replacement. Finally, under harsh conditions, the adhesive which affixes the tape to the rotor can fail.
Hydroelectric turbines and turbine blades or buckets can be eroded by impacting silt particles. Impingement, associated with bubbles entrained in a liquid contacting a surface at a high speed, can also erode hydroelectric turbines and turbine blades or buckets. Cavitation next to the surface of turbine blades or buckets can erode the surface of the turbine blade or bucket. In the past, the problem of impacting particles, impingement, and cavitation eroding fluid-handling parts exposed to a liquid, such as water, has been sought to be mitigated by the use of hard coatings, such as plasma-sprayed tungsten carbide. Although such coatings provide some protection to the fluid-handling part for a period of time, they are expensive and, on the whole, not very effective. For example, such hard coatings can transmit vibration associated with cavitation to the fluid handling part so that the fluid handling part is degraded.
When fluid-handling parts cannot be adequately protected from the effects of particle impact, impingement, or cavitation, the maintenance of these fluid-handling parts generally takes the form of replacement or repair of a worn part. However, the need for frequent replacement or repair has high associated labor and material costs. In certain applications, replacement may not be possible, e.g., it may not be possible to replace a helicopter rotor during a military operation. Repair of a composite rotor may be impractical. Repair of metal fluid-handling parts, such as turbine blades and buckets in hydroelectric turbines, may be possible but undesirable. For example, material used in welding can be different than the material from which the bulk of a fluid-handling part is formed and thus compromise the strength of the fluid-handling part or affect the structural properties in an undesirable way. It would be preferable if particle impact, impingement, and cavitation affected a coating which did not contribute to the strength of a fluid-handling part or substantially affect its structural properties so that the loss of or need to repair or replace the coating did not detrimentally affect the fluid-handling part.
The inadequate polyurethane tape is an example of a polymer coating. Certain other polymer coatings, such as polyurethane, provide some protection, but on the whole are impractical because of poor adhesion to fluid-handling parts and sensitivity to environmental factors such as hydrolytic degradation.
By contrast, silicone polymers have properties which can be advantageous in protecting fluid-handling parts. For example, silicone polymers are resistant to degradation by ultraviolet radiation, which is a positive characteristic for a material envisioned for coating helicopter rotors, which may be directly exposed to the sun for extended periods of time. Silicone polymers are not degraded by water, which allows them to be used for coating hydroelectric turbines. However, flexible silicone polymer coatings are infrequently used in applications where they must withstand severe mechanical stress, such as imposed by high-velocity impacting particles, in protecting machine or structural parts.
The prior art discloses the use of silicone coatings to protect polymer substrates from abrasion and weathering. However, the use of coatings to protect substrates against the effects of particle impact, impingement, and cavitation is not disclosed. The use of silicone coatings to protect metal substrates from electrochemical corrosion is disclosed in the prior art; however, the protection of metal substrates against the effects of particle impact, impingement, and cavitation is not disclosed. Other prior art discloses the use of coatings in delaying the onset of cavitation and absorbing vibration generated by cavitation, but the coatings themselves are not described as being resistant to erosion by particle impact, impingement, and cavitation, in the way taught by the present application.
There thus remains an unmet need for a method for using a silicone coating to effectively shield a fluid-handling part from erosion by impacting particles, impingement, and cavitation, the method being inexpensive and easy, and the coating providing protection for a prolonged period of time.